Institute for Health and Social Policy and Department of Epidemiology, Biostatistics, and Occupational Health, McGill University, Montreal, QC, Canada H3A
1A3; bInstitute on the Environment, University of Minnesota, St. Paul, MN 55108; cCollege of Resources and Environment, University of Chinese Academy of
Sciences, Beijing 100049, China; dEnvironmental Chemistry and Technology Program, Department of Civil and Environmental Engineering, University of
Wisconsin-Madison, Madison, WI 53706; and eMRC-PHE Centre for Environment and Health, Department of Epidemiology and Biostatistics, School of Public
Health, Imperial College London, London W2 1PG, United Kingdom

Air pollution in China and other parts of Asia poses large health
risks and is an important contributor to global climate change.
Almost half of Chinese homes use biomass and coal fuels for
cooking and heating. China’s economic growth and infrastructure
development has led to increased emissions from coal-fired power
plants and an expanding fleet of motor vehicles. Black carbon (BC)
from incomplete biomass and fossil fuel combustion is the most
strongly light-absorbing component of particulate matter (PM) air
pollution and the second most important climate-forcing human
emission. PM composition and sources may also be related to its
human health impact. We enrolled 280 women living in a rural
area of northwestern Yunnan where biomass fuels are commonly
used. We measured their blood pressure, distance from major traffic
routes, and daily exposure to BC (pyrolytic biomass combustion),
water-soluble organic aerosol (organic aerosol from biomass combustion), and, in a subset, hopane markers (motor vehicle emissions)
in winter and summer. BC had the strongest association with systolic
blood pressure (SBP) (4.3 mmHg; P &lt; 0.001), followed by PM mass
and water-soluble organic mass. The effect of BC on SBP was almost
three times greater in women living near the highway [6.2 mmHg;
95% confidence interval (CI), 3.6 to 8.9 vs. 2.6 mmHg; 95% CI, 0.1 to
5.2]. Our findings suggest that BC from combustion emissions is
more strongly associated with blood pressure than PM mass, and
that BC’s health effects may be larger among women living near a
highway and with greater exposure to motor vehicle emissions.
cardiovascular disease

| household air pollution | solid fuels

P

articulate matter (PM) air pollution is a leading health risk
factor (1) and primary contributor to anthropogenic climate
change (2). Air pollution is notoriously high in China and other
parts of Asia. China’s rising energy demands have led to increased air pollution emissions from coal-fired power plants (3).
Its motorized transport growth is the fastest in the world with the
number of motor vehicles projected to quadruple in the next two
decades, reaching over 380 million by 2030 (4). Meanwhile,
nearly half of all Chinese still cook and heat their homes with
highly polluting biomass and coal fuels (5). The resulting PM
concentrations routinely exceed the World Health Organization’s
(WHO) annual Air Quality Guideline of 10 μg/m3 by a factor of 10
or more (6) and are associated with a number of adverse health
outcomes, including cardiovascular diseases (1, 7).
PM differs in chemical properties, size, and possibly effects on
human health. Black carbon (BC) and organic carbon PM are
emitted during incomplete biomass and fossil fuel combustion
and seem to have important effects on both climate and human
health. BC affects the regional and global climate by absorbing
solar radiation and heating the atmosphere and is the second
most important climate-forcing human emission, after carbon
dioxide (8). Coemitted organic carbon may further influence
radiative forcing by acting as cloud condensation nuclei (9).
These specific characteristics and sources of PM may also impact
its toxicity to humans (10).
www.pnas.org/cgi/doi/10.1073/pnas.1317176111

We previously found that daily exposure to PM &lt;2.5 microns
in aerodynamic diameter (PM2.5) was associated with higher
blood pressure in older Chinese women cooking with biomass
fuels (11). In the current study, we used chemical and optical
methods to analyze the PM2.5 exposure samples for BC and organic components and evaluated their associations with blood
pressure, the leading risk factor for cardiovascular diseases,
worldwide and in China (1). We enrolled 280 women aged 25–90 y
who lived in six villages in the surrounding area along the
Yunnan–Tibet Highway in the Himalayan foothills of Yunnan
Province, China. These women were subsistence farmers and used
biomass fuels (largely wood and crop residues) for cooking and
space heating. Details about the study site, household fuel and
stove use patterns, and exposure to other PM sources like direct
and involuntary tobacco smoking are reported elsewhere (12).
The independent and combined effects of different PM components from various sources on human health are poorly understood. We used measurements of women’s daily PM exposure
and proximity to a highway to examine how PM composition and
sources affect the hazardous effects of blood pressure in this
group, using proximity to the highway as a proxy for exposure to
motor vehicle emissions using organic tracers.
Results
We enrolled 280 women (mean age: 51.9 y), 18% of whom were
overweight [body mass index (BMI) = 25–30 kg/m2] and 4%
obese (BMI ≥ 30 kg/m2). Mean systolic and diastolic blood
pressure (SBP and DBP) were 120 [95% confidence interval
(CI), 118 to 122] and 72 mmHg (95% CI, 71 to 73), respectively.
Significance
Air pollution is a leading health risk factor and important contributor to regional climate change in China and other parts of
Asia. China’s particulate matter (PM) air pollution dramatically
exceeds health guidelines and is impacted by industrial emissions, motor vehicles, and household use of biomass and coal
fuels. Black carbon (BC) from biomass and fossil fuel burning is
a major climate-forcing component of PM. We found that BC
exposure from biomass smoke is more strongly associated with
blood pressure than total PM mass, and that coexposure to motor
vehicle emissions may strengthen BC’s impact. Air pollution mitigation efforts focusing on reducing combustion pollution are
likely to have major benefits for climate and human health.
Author contributions: J.B., Y.Z., J.J.S., and M.E. designed research; J.B., Y.Z., W.H., and
Y.W. performed research; Y.Z. and J.J.S. contributed new reagents/analytic tools; J.B.,
W.H., and Y.W. analyzed data; and J.B., J.J.S., and M.E. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Freely available online through the PNAS open access option.
1

To whom correspondence should be addressed. Email: jill.baumgartner@mcgill.ca.

PM2.5 exposure in summer was 55 μg/m3 (range: 9–492) and
in winter was 117 μg/m3 (range: 22–634) (12). These levels
greatly exceed the WHO’s 24-h Air Quality Guideline of 25 μg/m3.
Geometric mean BC exposure was 5.2 μg/m3 (4 μg/m3 in summer
and 6 μg/m3 in winter; range: 2–44) which is lower than BC exposure in central New Delhi traffic (42 μg/m3) (13) but surpasses
daytime ambient BC in Beijing, Mexico City, and several cities
in Brazil (range: 1.9–4.8 μg/m3) (14–16). Exposure to watersoluble organic mass (WSOM), a more specific marker of primary
biomass smoke, was considerably higher than BC (geometric
mean of 12 μg/m3 in summer and 33 μg/m3 in winter; range:
1–352) (Table 1), indicating that women’s exposure is greatly
influenced by biomass combustion (17). Exposure to all pollutants was lower in the summer compared with the winter season;
however, the relative contribution of BC and WSOM fractions
to PM2.5 mass was approximately the same across seasons and
for different age groups (Fig. S1).
The PM components had a low-to-moderate correlation, with
higher correlations in summer than in winter (Table S3). The
largest correlation was between PM2.5 and WSOM (r = 0.67),
followed by PM2.5 and BC (r = 0.48), and finally BC and WSOM
(r = 0.44). The PM–BC correlation for rural Chinese women in
our study was lower than in outdoor air pollution studies in
North America and Europe and in urban Shanghai (PM–BC
correlation range = 0.50–0.90) (18, 19). Similarly, BC and
hopanes were barely correlated (r = 0.18) in our study, even
though both are emitted from motor vehicles. These correlations
were low possibly because women’s exposures were dominated
by a single emission source, namely biomass combustion, rather
than traffic emissions or an urban mixture of sources.
Among women living relatively close to the highway that
passed through our study site (i.e., less than the median distance
of 208 m in our sample), daily BC exposure was slightly higher
than those living farther away in winter, but the opposite occurred in summer (Table S4). Distance from the highway was not
strongly related to BC exposure, with each 100 m from the
highway associated with a 0.02 ln(μg/m3) lower BC exposure
(95% CI, 0 to 0.02; P = 0.30). In contrast, average exposure to
hopanes, specific tracers of motor vehicle exhaust in our study
setting, was significantly higher among women living in the village closest to the highway (median distance = 76 m) compared
with those in the village farthest from the highway (median
distance = 548 m) (4.6 vs. 1.1 ng/m3; P &lt; 0.001 for both seasons;
Table S4). In fact, the average near-highway hopane exposures
exceeded occupational levels among US trucking terminal
workers (4.6 vs. 1.9 ng/m3) (20).

Associations with Blood Pressure. We evaluated the associations of
SBP and DBP with exposure to PM2.5 mass, BC, and WSOM.
We express the results as the changes in SBP or DBP associated
with a 1-ln(μg/m3) increase in pollutant exposure using one and
two-pollutant multivariate mixed-effects models because there
was evidence of a nonlinear association (21, 22).
BC exposure had the largest independent effect on blood
pressure among rural Chinese women (Fig. 1). In models with
just one pollutant, a 1-ln(μg/m3) increase in BC was associated
with 4.3-mmHg higher SBP (P &lt; 0.001), followed by PM2.5 mass
(2.2 mmHg; P = 0.002) and WSOM (1.2 mmHg; P = 0.06). The
estimated effect of BC on SBP changed little (7% change) and
remained statistically significant after including PM2.5 mass or
WSOM in the model. In contrast, the estimated effect of PM2.5
mass on SBP decreased by 77% and was no longer statistically
significant when BC exposure was added to the model. WSOM
had no effect on SBP after other PM components were included
in the model (Fig. 1A). We found the same strong and statistically robust relationship between BC exposure and DBP (Fig.
1B). Our conclusions remained the same when evaluating the
changes in blood pressure associated with an interquartile range
(IQR) increment in log-transformed pollution exposures, although the difference in the estimated effect of BC on blood
pressure relative to PM and water-soluble organic carbon
(WSOC) was slightly reduced. An IQR increase in ln(BC) had
the strongest and most robust association with higher SBP
(3.6 mmHg; 95% CI, 2.0 to 5.2), followed by ln(PM2.5) (2.8 mmHg;
95% CI, 1.0 to 4.6) and ln(WSOM) (1.8 mmHg; 95% CI, 0.1 to
3.5) (Fig. S2).
We conducted a separate analysis for younger (25–50 y) vs.
older women (&gt;50 y). BC was more strongly associated with
blood pressure than PM mass among both younger and older
women. A 1-ln(μg/m3) increase in BC exposure was associated
with a 1.8-mmHg (95% CI, 0 to 3.6) higher SBP in younger
women at the sample average, compared with no effect for
PM2.5. Among women &gt;50 y old, a 1-ln(μg/m3) increase in BC
was associated with a 7.4-mmHg (95% CI, 4.0 to 10.8) higher
SBP and a 2.9-mmHg (95% CI, 1.1 to 4.7) higher DBP (Fig. 2).
Our conclusions did not change when evaluating the changes in
blood pressure associated with IQR increases in ln(BC) and ln
(PM) exposures by age. The effect of an IQR increase in ln(PM)
on blood pressure was similar to models estimating a 1-ln(μg/m3)
change in pollution, while the effect of the IQR change in ln(BC)
was slightly reduced. In older women, an IQR increase in ln(BC)
was associated with a 6.2-mmHg (95% CI, 3.4 to 9.0) higher SBP
and a 2.4-mmHg (95% CI, 0.9 to 3.9) higher DBP. In comparison, an IQR increase in ln(PM) was associated with a 5-mmHg
high SBP (95% CI, 1.9 to 8.1) and a 2.2-mmHg (95% CI, 0.5 to
3.9) higher DBP. (Fig. S3).
The relatively consistent pollution ratios across age groups and
seasons (Fig. S1) suggest that stronger blood pressure effects of
pollution in older women are not a result of age-specific differences in PM composition. Excluding women taking hypertensive
medication from the analysis did not change our estimated

*The sample size (n) refers to the 24-h exposure sample for a woman enrolled in our study. Missing values are for
filters that were damaged during optical or chemical analysis (∼1% of samples). GM, geometric mean; Min–max,
minimum to maximum.

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Baumgartner et al.

A

2.2
(0.8, 3.6)

PM2.5
alone

2.3
(1.0, 3.6)

PM2.5
PM2.5
with BC with WSOM

-0.1
0.4
0.5
(-0.3, 1.3) (-1.1, 0.9) (-0.9, 1.7)

4.0
4.3
(2.3, 6.3) (1.7, 6.3)

4.6
(2.3, 6.9)

BC
BC
BC
alone with PM2.5 with WSOM

1.4
1.3
(0.2, 2.4) (0.0, 2.8)

1.5
(0.1, 2.9)

-0.2
-0.3
1.2
(-0.1, 2.5) (-2.2, 1.8) (-1.8, 1.2)

WSOM
WSOM
WSOM
alone with PM2.5 with BC

0.0
-0.3
0.3
(-0.4, 1.0) (-1.1, 1.1) (-1.1, 0.5)

SUSTAINABILITY
SCIENCE

B

0.5
(-1.2, 2.2)

PM2.5
alone

PM2.5
with BC

PM2.5
with WSOM

BC
BC
BC
alone with PM2.5 with WSOM

WSOM
WSOM
WSOM
alone with PM2.5 with BC

Fig. 1. Associations of personal exposure to PM mass, BC, and WSOM on (A) SBP and (B) DBP using one- and two-pollutant mixed-effects regression models.
ΔSBP or ΔDBP represent the difference in SBP or DBP (with 95% CIs) associated with a 1-ln(μg/m3) increase in pollutant exposure.

posure had a larger effect on blood pressure among women
living closer (i.e., &lt;208 m from the highway) to the highway that
passes through our study site than those living farther away. In
the former group, a 1-ln(μg/m3) increase in BC exposure was
associated with almost threefold higher SBP than in women
living away from the highway (6.2 mmHg; 95% CI, 3.6 to 8.9 vs.
2.6 mmHg; 95% CI, 0.1 to 5.2; interaction P = 0.04). The effect
of BC exposure on DBP was also noticeably larger among
women living near the highway (2.6 mmHg; 95% CI, 1.0 to 4.2
vs. 0.3 mmHg; 95% CI, −1.3 to 2.0; interaction P = 0.02). The
3.6-mmHg larger effect of 1-ln(BC) on SBP among women near
the highway is similar to the SBP impact of a modest reduction in
sodium intake (−4.2 mmHg per 4.4-g daily reduction) (23) and
within the range of an SBP decrease with use of antihypertensive
medication (−2.5 to 12 mmHg) (24).
Highway proximity was also a predictor of the effect of BC on
blood pressure at the village level, with the effect strongest
among women living in villages near the highway and absent
among those living in the two villages farthest away from the
highway (Fig. 3). Distance from the highway was not independently related to blood pressure, nor did the relationship between
Baumgartner et al.

PM2.5 or WSOM exposure and blood pressure differ by distance
from the highway (all interaction P values &gt;0.80).
Discussion
We found that BC exposure was more strongly associated with
blood pressure than PM2.5 mass and WSOM among Chinese
women cooking with biomass, and that the effect was stronger
among women living near the highway and with greater exposure
to motor vehicle emissions. These findings provide several important directions for future health effects studies and for policies and other mitigation strategies aimed at reducing air
pollution emissions and exposures.
First, BC may be a useful indicator of the cardiovascular health
and climate benefits of interventions that lower air pollution
concentrations and exposures. Air pollution mitigation efforts
and guidelines, including those in China, have traditionally focused
almost exclusively on PM rather than its components or sources
(25). However, different interventions may affect PM components,
like BC, by varying amounts. For example, some so-called improved cookstoves have emitted higher BC concentrations than
traditional stoves, even if they reduced PM2.5 mass (26, 27). Not
all mitigation options that reduce PM emissions will also reduce
total climate forcing or, potentially, benefit health. The inclusion
of BC as an outdoor air quality indicator has been proposed (18,
PNAS Early Edition | 3 of 6

A 1110
9
8
SBP (mm Hg)

7
6
5
4
3
2
1
0
-1

B

5

DBP (mm Hg)

-2

3

4
2
1
0
-1
-2
-3

BC
BC PM PM
alone with alone with
PM
BC
23 – 50 years

BC
BC
PM
PM
alone with alone with
PM
BC
&gt;50 years

BC
BC
PM PM
alone with alone with
PM
BC
All ages

Fig. 2. Associations of personal exposure to PM2.5 mass and BC on (A) SBP
and DBP (B) using one- and two-pollutant mixed-effects regression models,
by age. ΔSBP or ΔDBP represent the difference in SBP or DBP (with 95% CIs)
associated with a 1-ln(μg/m3) increase in pollutant exposure.

28) but not adopted, and current guidelines for evaluating biomass cookstoves focus on PM2.5 and carbon monoxide (29).
Second, BC could be an important exposure assessment tool
for future health studies. The larger magnitude of blood pressure
response and extension of the health impact to younger women
strengthens the importance of our initial finding on PM2.5 exposure and blood pressure. Our results support a recent metaanalysis of studies in US and European cities showing that
ambient BC concentrations were more strongly associated with
cardiovascular mortality and hospital admissions than PM mass
(18). If BC is more strongly linked with health than PM, its measurement will facilitate smaller sample sizes and more accurate
estimate health impacts of air pollution, and of interventions and
policies. There are several methods to measure BC, including
simple and low-cost optical assessment on existing PM filter samples
or real-time measurement using a new generation of lightweight
personal samplers. The inclusion of BC in studies already measuring
PM2.5 requires less additional effort and resources compared with
other combustion markers like WSOM or organic hopanes.
Finally, we found an indication that the cardiovascular effect
of BC from biomass smoke may be stronger if there is coexposure to motor vehicle emissions. Our results demonstrate that
the blood pressure effect of BC observed in the United States
and Europe (30, 31) is not limited to high-income countries
where BC is primarily from motor vehicles, although residential
biomass combustion contributes to winter ambient air pollution
in northern climates (32–34). The stronger health effect of BC
from roadway exposure or combined roadway–biomass exposure
may also be an important environmental risk factor for cardiovascular diseases in developing countries like China where the
number of motor vehicles is rapidly increasing and household
use of biomass and coal fuels persists (5, 35).
There are several possible reasons for the stronger effect of
BC on blood pressure. One is that BC more closely identifies PM
from combustion sources than heterogeneous PM mass does,
which comprises particles from all sources, or WSOM, which is
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both emitted as a primary pollutant and formed as a secondary
aerosol. Toxicological studies indicate that PM from incomplete
combustion may be more toxic in macrophage and fibroblast cell
lines than PM from more complete combustion (36, 37). It is also
possible that other components of combustion-related PM contribute to the observed health impacts, with BC acting as a surrogate for their levels. In vitro studies indicate possible toxicity of
certain organic constituents in PM from biomass combustion and
suggest that BC may be a carrier of these compounds for uptake
into macrophages and epithelial cells (36, 38). BC may be operating
as an indicator for a larger category of primary combustion particles
with varying toxicity to humans, which, in addition to BC, could
include metals or polycyclic aromatic hydrocarbons, any of which
could act individually or in combination to increase blood pressure
(39). Although we cannot determine the single or combination of
PM components responsible for the stronger BC effect in our study,
our results suggest that a reduction in PM exposure containing BC
and other combustion-related particles for which BC is an indicator
should lead to a reduction in the adverse health and climate impacts
of air pollution.
Our study is limited by its cross-sectional design. However,
cooking with biomass is a long-term behavior and all residents
have lived in their current homes throughout their adult lifetime.
Thus, 24-h PM exposure is a measure that is typical except for
seasonal and day-to-day variability; we found little variability in
the relative composition of PM exposure by season or daily
patterns for women in our study. We considered that factors
associated with both blood pressure and highway proximity (e.g.,
excessive noise or stress from living near the highway) might
explain the stronger effect of BC among women closer to the
highway. However, highway proximity did not affect the relationship between PM2.5 mass and blood pressure, suggesting
that distance from the highway is not a proxy for these potentially confounding variables given that PM2.5 exposure is also
associated with both motor vehicle emissions and blood pressure.
Our results are consistent with several intervention studies in
Latin America that found a decrease in SBP (∼3–6 mmHg) in
older women who switched from a traditional open fire cookstove to a less-polluting chimney stove (40, 41). Although blood
pressure is an important risk factor for cardiovascular diseases
and overall global burden of disease (1), further research should
assess the associations of BC with other health outcomes,
including those in vulnerable populations like children. Our study is

Fig. 3. Associations of personal black carbon exposure and blood pressure
(in mm Hg), by village. Estimates and 95% confidence intervals are the
changes in SBP and DBP (in mm Hg) associated with a 1-ln(μg/m3) increase in
black carbon exposure, by village. The six villages are outlined in red or blue
and the yellow line denotes the Yunnan-Tibet highway. DBP, diastolic blood
pressure; SBP, systolic blood pressure.

Baumgartner et al.

Conclusion
Our results show that the effect of BC exposure on blood pressure is two or more times larger that of PM2.5 mass and WSOM
among rural Chinese women using biomass fuels. We also found
evidence that BC from biomass smoke is associated with higher
blood pressure in the presence of motor vehicle emissions as
a coexposure. Our findings suggest that BC has direct relevance
as an important environmental risk factor for cardiovascular
diseases and support the use of BC as a pollution indicator in
future health studies and in the evaluation of air pollution mitigation programs. More broadly, our results may be useful in
forming policy aimed at reducing air pollution and improving public
health in China and other developing countries. China recently
committed to spending US$275 billion over the next 5 y to reduce
air pollution (44), but targets for new vehicle emission standards are
absent from recently announced mitigation plans (45). In addition,
China’s current air pollution targets and programs focus on PM
reductions. The BC reduction achieved with any mitigation strategy
is not always proportional to the reduction in PM mass, and our
results show that BC may be more strongly associated with health
outcomes in addition to warming the climate. As motorized transport and subsequent traffic emissions increase throughout China,
air pollution policies and mitigation efforts that focus on BC control
might have the largest benefits for climate and human health.
Materials and Methods
We recruited 280 women ≥25 y old between December 2008 and August
2009. None were previous or current tobacco smokers. Families in this region
had similar diets, lifestyles, and socioeconomic backgrounds. The Institutional
Review Boards at the University of Wisconsin–Madison and Yunnan Provincial
Health Bureau approved this research protocol and obtained informed oral
consent was obtained from all participants.
Personal Air Pollution Exposure Measurements. We measured the participants’
24-h exposure to PM2.5 on 1–3 consecutive sampling days in winter and
summer. Participants wore a waistpack holding lightweight air samplers that
collected PM2.5 on Teflon filters. They were instructed to perform routine
daily activities while wearing the waistpack, but could place it within 1 m
while sitting or sleeping and within 2 m while bathing. Field staff monitored
compliance through home visits.
PM2.5 mass was gravimetrically estimated on all exposure samples and
blanks using a high-precision microbalance in a temperature and humidity
controlled room. They were then analyzed for BC, WSOC, and mobile source
tracers at the University of the Chinese Academy of Sciences. We estimated
BC concentrations using reflectance analysis with an Optical Transmissometer
Data Acquisition System (Model OT21; Magee Scientific) (46).
We used the nonpurgeable organic carbon method described in Timonen
et al. (47) to estimate WSOC concentrations. Briefly, filter sections were
extracted with high purity water and analyzed with an organic carbon analyzer (Shimadzu Corp.). The resulting WSOC exposures were multiplied by
2.0 to yield WSOM from biomass burning (48, 49). For exposure samples
from women in the village either nearest to (n = 32 women) or farthest from
the highway (n = 53 women), we analyzed the remaining filter sections for
hopanes, nonpolar organic tracers of motor vehicle exhaust, using the extraction-derivatization method with GC/MS described by Zhang et al. (50).
Details about these PM components, their measurement, and related
quality assurance and control practices are described in Baumgartner et al.
(12) and discussed in SI Text.

Physiological and Health Parameters. Initial questionnaires evaluated household demographics, socioeconomic status, secondhand-smoking status, and
medical history. We conducted in-home blood pressure measurements using
an automated device and following standard recommendations (51). We
recorded the time of measurement, air temperature, and any caffeine
consumption in the previous hour. We measured each participant’s height
(centimeters), weight (kilograms), waist circumference (centimeters), and
salt intake from cooked foods and used a pedometer to assess 24-h physical
activity. Details on measurement of blood pressure and the other health and
sociodemographic factors related to blood pressure are described elsewhere
(11) and provided in Tables S1 and S2.
The location of each participant’s home was georeferenced using aerial
photographs of the study villages from Google Earth (52). We calculated the
shortest distance between each participant’s home and the closest highway
segment to determine the distance from the highway. Smaller roads within
villages are narrow and mainly used for walking.
Analysis. We estimated the geometric means and ranges of exposure to PM2.5
mass, BC, WSOM, and hopanes by season and age group. Spearman correlation analysis was used to assess collinearity between nonnormally distributed pollution components.
We analyzed the differences in blood pressure associated with 1-ln(μg/m3)
unit increases in pollution exposure using one- and two-pollutant mixedeffects models to determine if observed associations in one-pollutant models
were robust to the inclusion of a second pollutant. We only included the first
day of pollution exposure in each season because blood pressure was not
measured after subsequent second and third days of PM exposure assessment. The two-pollutant models may also help distinguish between the
blood pressure effects of combustion vs. noncombustion pollution (BC vs. PM
mass), biomass vs. other combustion pollution (WSOM vs. BC), and biomass
vs. other nonbiomass pollution (WSOM vs. PM mass).
We used multivariate regression models from our previous study on PM2.5
exposure and blood pressure (11) so that any differences in the results could
be unambiguously attributed to the difference in pollution variables. The
following variables were included in all regression models: age, waist circumference, physical activity (daily number of steps), socioeconomic status,
daily salt intake, day of the week and time of day of blood pressure measurement, and ambient air temperature. Passive tobacco smoking, education, caffeine intake, and self-reported health were neither associated with
blood pressure at P &lt; 0.10 nor did they change the effect of pollution exposure on blood pressure at ≥10%, and were therefore excluded from the
final models.
In a second analysis, we allowed the effect of pollution exposure on blood
pressure to vary by village using the following model:

yifkj = μ + βifkj + ζ k βkj + γxifkj + ηzifk + aifk + hfk + vk + «ifkj ,
where yifkj is SBP or DBP for individual i in household f in village k at time j,
βifkj is an individual’s exposure, ζ k(βkj) is the random slope at the village level,
xifkj are other individual-level covariates that vary seasonally (e.g., ambient
air temperature); zifk are covariates (e.g., socioeconomic status); aifk is a random intercept; hfk and vk represent the random effects to account for correlation at the household and village levels, respectively; and eifkj is the residual.
For sensitivity analyses, we used a scale of the IQR on log-transformed
exposures to facilitate comparison between pollutants with different concentration ranges. To assess the validity of the distance from the highway as
a proxy for exposure to motor vehicle emissions, we conducted two-sample t
tests on geometric mean hopane exposure among women living in the villages nearest to and farthest from the highway. We also conducted a separate analysis excluding women who reported taking antihypertensive
medication, all of whom were &gt;50 y old.
All statistical analyses were performed in STATA 11 (StataCorp LP).
ACKNOWLEDGMENTS. We thank Arden Pope, Brian Robinson, and Gerard
Hoek for valuable comments on early results; our field staff in Yunnan
for their hard work; and the Lashihai residents for allowing us into their
villages. We are grateful for the support of University of the Chinese
Academy of Sciences Hundred Talents of the Chinese Academy of Sciences
Grant Y12901FEA2 (to Y.Z.) and the Initiative for Renewable Energy and
the Environment at the University of Minnesota’s Institute on the Environment (J.B.).

also limited by proxy measurements, using distance, of motor vehicle exposure in the full sample. Distance from the highway has
nonetheless been used as an indicator of roadway air pollution
exposure in developed countries (42, 43). Further, women living
nearest to the highway had significantly higher exposure to hopanes,
direct markers of motor vehicle emissions, than women farthest
from the highway in our subsample tracer analysis.